1. Field of the Invention
The invention relates to an apparatus for precision positioning accomplished through sophisticated electronic control of electromagnetic actuation. The apparatus finds particular application to position read/write heads in rotating magnetic direct access storage devices.
2. Description of the Prior Art
Devices utilized for accurate and repeatable positioning are of extreme industrial importance. For example, photomasking as utilized for the manufacture of large scale integrated circuits requires the establishment of conductive and nonconductive areas on a mask with tolerances on the order of microinches. Mechanisms utilizing lead screws and voice coil motors have been used in such applications, but the hardware and/or control circuitry associated with their use is expensive and cumbersome. In another example, both analog and digital plotters that are used to create graphical representations upon a printing medium or paper with electrically varying input signals have reached a point where the mechanisms utilized for positioning the pen or print head with sufficient accuracy are expensive and cumbersome. As another common example, rotating magnetic memories utilizing magnetic disks require actuators to move read-write recording transducers or heads radially so as to permit the read-write heads to record data upon or extract data from the recording medium. The actuators for moving the recording transducers have positional accuracy requirements on the order of microinches. Such actuators must be inexpensive to manufacture, of extreme reliability, and of repeatable accuracy. Disk drive actuators of the voice coil motor type and the lead screw type are known in the art, but the objective of ever increasing precision and decreasing cost of manufacture of such disk drives would be well served by an electromagnetic actuator of extreme reliability, low cost, and simplicity of design.
One type of electromagnetic actuator which has been utilized in some precision positioning applications, including data processing applications, is a mechanism in which the primary motion-producing element is a solenoid. Solenoids of the type consisting of a cylindrically symmetrical structure in which a plunger is made to move under the urging of a magnetic field induced by a coil with suitable electrical excitation are old in the art. Simple solenoids, however, are inadequate for high-accuracy applications as special design techniques must be employed to prevent wear, nonrepeatable errors, and component failure. Moreover, a simple solenoid has merely two positions, "on" and "off", which are not suitable for multipositioning applications.
The art of solenoid positioning, however, has advanced beyond a simple "on" and "off" device. U.S. Pat. No. 3,219,854 issued to McLaughlin discloses a linear actuator constructed from a plurality of electromagnetic solenoids formed into a cylindrical stationary housing having a longitudinal bore. The bore contains a plurality of slidable plunger-core assemblies (i.e., solenoids) which are connected in series to each other and which have one end fixed to the housing and the other end attached to an output shaft. The coil of each solenoid is individually connected to a source of electrical excitation. Each solenoid operates within the longitudinal bore of the cylindrical stationary housing against its adjacent solenoid or plunger-core assembly. Moreover, the stroke of each individual solenoid is chosen to be twice the stroke of the immediately preceding solenoid. In this manner, the entire stroke of the plurality of series-connected plunger assemblies may be divided in a binary fashion so as to define discrete digital positions.
McLaughlin also teaches the use of helical coil springs between each plunger-core assembly. The helical coil springs are used to reset the plunger-core assemblies when electrical excitation is removed from a plunger-core assembly; that is, the helical coil springs are the returning force for the individual solenoids. Each helical coil spring is specified in McLaughlin as having a strength slightly less than the pulling force capable of being exerted by an energized solenoid.
U.S. Pat. No. 3,275,964 issued to Kumm discloses another electromagnetic actuator device. Kumm discloses an apparatus constructed from two solenoid sections connected and cooperating with one another along a single axis. A first section consists of a pair of solenoids operating in tandem which may be selectively energized to move an output shaft. Only upon actuation (i.e., excitation) of the first solenoid pair may the second section of solenoids be actuated. The second section of solenoids consists of three solenoids adjacent to each other and disposed about a single shaft. By appropriate excitation of the three solenoids, any of eight different positions of the output shaft may be chosen.
U.S. Pat. No. 3,430,120 issued to Kotaka et al is directed to an electromagnetic positioning mechanism. The apparatus is constructed from a plurality of plunger-type electromagnets which are arranged in side-by-side fashion so as to selectively control a plunger member which is arranged to be reciprocally moved therebetween. The Kotaka mechanism uses a multiplicity of electromagnetic actuators in serial fashion so as to choose a desired output stroke.
U.S. Pat. No. 3,491,319 issued to Cox et al discloses a digital actuator constructed from a plurality of coaxial and abutting piston adder assemblies arranged in a columnar stack. Each piston adder assembly includes a pair of coaxial cylindrical elements slidable relative to each other along their common axis. Separate spring elements are utilized to hold these individual piston adder assemblies in a normally expanded and spaced apart relationship whereby the position of an output shaft is controlled by the sum total of the linear extensions of each individual piston adder assembly. Selective contraction of designated adder assemblies can choose a variety of output shaft strokes. The structure of Cox et al bears some similarity to the structures disclosed in McLaughlin and Kumm discussed hereinabove. Cox et al again discloses the concept of a digital mechanical actuator in which the strokes of individual solenoids are chosen to be digitally related to each other. Individual spring elements are utilized to hold individual piston adder assemblies in an expanded condition.
It is thus clear that the juxtaposition of a plurality of electromagnetic plunger-core assemblies into a structure wherein the stroke of each individual plunger-core assembly is chosen to be digitally related to the other plunger-core assemblies so as to achieve a digital output is old in the art.